Dispersion process

ABSTRACT

A continuous process is disclosed for preparing a reactive particulate dispersion in a liquid carrier. The process comprises mixing together as a melt a resin and a crosslinker under extrusion condition at a temperature and for a time such that substantial crosslinker potential is retained between the resin and the crosslinker and thereafter dispersing the mixture whilst still molten into the liquid carrier and allowing the molten dispersed mixture to solidify to form particles.

This is a continuation of application Ser. No. 09/180,566, filed Nov. 6,1998, now U.S. Pat. No. 6,204,310, a 35 USC 371 national filing based onPCT/EP97/02841, filed May 26, 1997, the benefit of which is claimed.

This invention relates to a process for preparing reactive particulatedispersions and reactive powders, in particular a process formanufacturing aqueous dispersions of suspended particulates in acontinuous process.

A reactive particle consists of a resin (that is to say a natural orsynthetic polymer having reactive functional groups) and a crosslinker(that is to say a monomer, an oligomer or other polymer having reactivefunctional groups capable of reacting with the reactive functionalgroups on the resin) which can be reacted together to form a crosslinkednetwork. In a reactive particle a proportion of the functional groups inthe resin and the crosslinker will be un-reacted, and be capable ofundergoing crosslinking reactions with functional groups in resin andcrosslinker molecules in the same or other reactive particles. It isalso a feature of such particles that they will be an intimate mixtureof ingredients, with each particle having essentially the samecomposition.

Reactive particulate dispersions are typically used in the preparationof coating compositions where, following application to a surface to becoated, any carrier liquid is either allowed to evaporate, or will bedriven off, and the particles caused to form a crosslinked film, forexample by stoving. Reactive powders are used in powder-coatings wherethe dry powder is applied to a surface to be coated, and then caused tocrosslink in the same way.

Conventionally, powder coatings are prepared by melting together a resinand a crosslinker for a time and at a temperature such that nosubstantial reaction takes place between them. The melt is thenextruded, allowed to solidify as a mass, and the solid extrudate is thenpulverised, with the powder so obtained then being classified. Such apowder can be applied directly or as a dispersion in a liquid carrier.However, a problem with such processes is the need for the grinding andclassification steps, which are relatively expensive. In particular, thegrinding step requires the use of expensive cooling equipment,particularly with low Tg polymers, to ensure both that the temperatureof the resin does not exceed that at which further reaction may occur,and also to ensure that the resin remains brittle, in order thatsatisfactory particle size reduction can occur.

It is also a disadvantage of such a process that the resin particles soproduced tend to have irregular particle shapes and sizes, and thatthere is poor control over these parameters. Regular particle shape andsize is advantageous, as is the provision of substantially sphericalparticles, since such particles can have advantages in terms of theirability to retain electrostatic charge when they are used in particlecoatings. They can also be more fluidizable.

It is also known to produce certain types of dispersions for use aswater based paints. For example, U.S. Pat. No. 5,087,645 (Toyo SeikanKaisha Ltd.) discloses a batch process for producing a water based paintcomposition which comprises an epoxy resin, a curing agent and anacrylic resin, wherein the acrylic resin has an acid number between 2and 30, and wherein the acrylic resin has been modified by convertingthe carboxyl groups on it to amine or ammonium salts, by the addition ofan aqueous solution of ammonia or an amine. The resulting solutionundergoes phase inversion to provide the water based paint.

We have now discovered that reactive dispersions and powders can be mademuch more simply, from a wider range of materials, and to a higherdegree of uniformity by a melt/dispersion process which is carried outon a continuous basis (i.e. preferably in an extruder), and also hascertain other advantageous aspects.

Accordingly the present invention provides a continuous process forpreparing a reactive particulate dispersion in a liquid carrier whichprocess comprises mixing together under extrusion conditions to form amolten mixture a resin and a crosslinker under shear at a temperatureand for a time such that substantial crosslinking potential is retainedbetween the resin and the crosslinker, and thereafter dispersing themixture whilst still molten into a liquid carrier, and allowing themolten dispersed mixture to form particles.

By carrying out the process under extrusion conditions, this allows theoperator to heat and mix rapidly the components of the composition, butalso to rapidly cool the mixture once an intimate blend is attained,thereby minimising the extent of reaction that occurs. It is highlypreferred that the reaction is carried out in an extruder, and alsohighly preferred that the extruder is a twin screw extruder, to ensurethat the desired temperature control and intimate mixing is achieved.

According to a further aspect of the invention, there is provided acontinuous process for preparing a reactive particulate dispersion in aliquid carrier in an extruder equipped with a main intake, an exit port,and an intermediate liquid injection port between the main intake andthe exit port, and heating means for heating material as it passesthrough the extruder between the main intake and the liquid injectionport, in which a resin and a cross linker are introduced into theextruder through the main intake, and are heated and mixed together asthey pass through the extruder so as to form a molten mixture beforethey reach the liquid injection port, and a liquid carrier is introducedinto the extruder through the injection port, and the molten mixturebecomes dispersed in the liquid medium, the temperature and thethroughput of the extruder being such that substantial crosslinkingpotential between the resin and the crosslinker remains in the finaldispersion, and the dispersion is allowed to cool on leaving theextruder such that the molten mixture solidifies to form a particulatedispersion.

Preferably, where the reactive particulate dispersion is intended foruse as a coating composition, other coating composition components suchas pigments, flow agents and catalysts can be introduced into the mainintake at the same time as the resin and the crosslinker. Preferably thecomponents are subjected to a short physical pre-mixing step at ambienttemperature prior to being introduced to the extrusion conditions. Also,where a dispersion agent is used, it is preferred that this isintroduced into the injection port at the same time (e.g. predispersed)with the liquid carrier.

A highly preferred aspect of the process according to the presentinvention is that it can be carried out without the addition of, and inthe absence of any solvents which are capable of dissolving the resin inthe composition, such as volatile organic solvents. This provides aclear advantage of the process over other known processes, such as forexample that described in the above mentioned U.S. patent (which inpractice requires the use of volatile organic solvents). In particular,the use of such solvents can cause the Tg of the resin to decrease, andalso decreases the blocking temperature of the final powder composition.This can lead to unacceptable agglomeration of the resin in the powderstate, making it less suitable for use. Preferred coating compositionsdo not contain any solvent which is capable of dissolving the resin inthe composition, and also have not had any such solvent used in theirmanufacture, as in practice if such a solvent has been used in theirmanufacture, some is inevitably retained in the resin. This leads to theundesirable attributes described above. As such, preferably the blockingtemperature of the resin powder which can be isolated from thedispersion produced according to the invention is at least is 40° C.

Consequently, it is preferred that the liquid carrier used for thedispersion step is not capable of dissolving the resin in thecomposition, and is therefore immiscible with the polymer components ofthe reaction mixture. The use of such an immiscible liquid carrier wouldnot significantly reduce the viscosity of the polymer in the dispersionbeing processed, and also would not significantly affect the Tg of thedispersed polymer resin.

An advantage of the process according to the invention is that itprovides for the preparation of polymer dispersions that have a higherreactivity (i.e. pre-disposition towards cross-linking) than other knowndispersions. The resulting polymer dispersions, which can be dried byremoval of the carrier liquid, also provide free flowing powders and canprovide powders which have substantially spherical particles. This firstcharacteristic is a function of the process being used to produce thepolymer not involving any volatile organic compounds or solvents whichare miscible with the polymer.

An important feature of the process according to the invention is theselection of the appropriate process conditions in the extrusionprocess, which is preferably carried out in an extruder and morepreferably a twin screw extruder, so as to provide a polymer dispersionwhich has a relatively high reactivity. This can be established byroutine experimentation, but as a generalisation the temperature ofprocessing in an extruder can be higher than could for example be usedin the process according to U.S. Pat. No. 5,087,645, but should not beso high as to cause the polymer to react excessively so as to depletethe reactivity of the polymer. The processing temperature can also becontrolled in conjunction with the residence time in the extruder, againso as to optimise the conditions for the production of the polymer.

Advantages accrue to the continuous (i.e. single step) process of theinvention in this respect, since the use of an extruder to provide acontinuous process provides better control over the process parametersso as to optimise the properties of the polymer dispersion. The use ofan extruder also means that under the intimate conditions of shearing,and rapid homogenisation which occurs, residence times and reactiontemperatures can be minimised so as to minimise the unwanted reaction,and maximise the degree of residual reactivity of the polymer duringproduction of the dispersion. Conveniently, the step of introducing animmiscible carrier liquid to provide the polymer dispersion can act as acooling step at the end of the process.

A further advantage of the process according to the invention is thatthe particles of dispersed polymer produced tend to be substantiallyspherical. This is in contrast to the relatively irregular acicularparticle shape produced in a grinding and classifying regime.

The particles produced in the process according to the invention aretypically in the range 1-50 microns, which makes them particularlysuitable for use in the production of powder coatings.

With regard to suitable polymers, in principle any resin/crosslinkersystem that can be used to make powder coatings can be used in theprocess of this invention. Examples of such systems are acid/epoxysystems, acid anhydride/epoxy systems, epoxy/amino resin systems,polyphenol/epoxy systems, phenol formaldehyde/epoxy systems, epoxy/aminesystems, epoxy/amide systems, isocyanate/hydroxy systems,carboxy/hydroxyalkylamide systems and hydroxyl/epoxy systems. Optionallythe systems can also contain a catalyst or blocking groups. Such systemsare described in Powder Coatings Chemistry and Technology, T. A Misev,John Wiley & Sons Ltd 1991.

In a polyphenol/epoxy system the resin generally consists of apolybisphenol A resin which has a molecular weight in the range 1,000 to20,000, more particularly 1,000 to 10,000 and especially 1,000 to 4,000.Examples of such resins are Araldite resins which are commercialavailable. Particular resins are Araldite GT-7072 and Araldite GT-7220.In such systems the crosslinker is a polyepoxide. This will have amolecular weight in the range 1,000 to 10,000, particularly 1,000 to4,000. In particular it will also be a novolac-type polyepoxide. Novolacpolyepoxides are commercially available. Polyphenol/epoxy systems can becatalysed with for example 2-methyl imidazole.

In an isocyanate/hydroxy system the hydroxy resin can be an hydroxyfunctional polyester or an hydroxy functional acrylic resin.

Hydroxyl functional polyester polymers have units derived from one ormore polybasic acids and units derived from one or more polyhydroxycompounds.

Polybasic acids are compounds having two or more carboxylic acid groupsor an equivalent number of anhydride groups (on the basis that oneanhydride group is equivalent to two acid groups). Such polybasic acidsare well known in the polyester art. Examples of suitable polybasicacids are C₁₋₆ alkane dioic acids such as adipic acid or hexanedioicacid, cycloaliphatic acids such as hexahydrophthalic acid, unsaturatedalkane dioic acids such as fumaric or maleic acids, dimer acids, andaromatic acids such as phthalic acid. Their equivalent anhydrides suchas maleic anhydride or phthalic anhydride can also be used.

Polyhydroxy compounds are compounds having two of more hydroxyl groupsand are well known in the polyester art. Examples of suitablepolyhydroxy compounds are trimethylol propane, glycerol, neopentylglycol and pentaerythritol.

The hydroxy functional polyester polymer are commercially available (egUralac P2115 and P2504 from DSM Resins BV) and can be produced by knownmethods.

Hydroxy functional acrylic addition polymers are derived frompolymerisable ethylenically unsaturated monomers such as vinyl oracrylic monomers and comprise functional units and structural units.

Whenever referred to herein, the term acrylic monomer refers to estersof acrylic or methacrylic acid. The term (meth) acrylate means acrylateand methacrylate. The term (meth) acrylic acid means acrylic andmethacrylic acids. Functional units are derived from hydroxy functionalvinyl or acrylic monomers. Examples of hydroxy functional acrylicmonomers are hydroxyethyl (meth) acrylate, hydroxybutyl (meth) acrylateand hydroxypropyl (meth) acrylate. A preferred hydroxy functionalmonomer is 2-hydroxyethyl methacrylate.

Examples of other hydroxy functional monomers are the reaction productsof glycidyl methacrylate with monocarboxylic acids.

Structural units are derived from monomers which do not have hydroxylgroups and will not react with the crosslinker. Examples arenon-functional vinyl monomers and alkyl esters of (meth) acrylic acid.Examples of non-functional vinyl monomers are styrene and 2-phenylpropene. Examples of alkyl esters of (meth) acrylic acid are C₁₋₁₂ alkylesters such as methyl (meth) acrylate, ethyl (meth) acrylate, n-butyl(meth) acrylate, t-butyl (meth) acrylate and n-propyl (meth) acrylate.

Examples of an hydroxy functional acrylic addition polymer are Elvacite2810 (Mn 7000, OH value 64 mg KOH/1 g resin; from ICI) and NeocrylXB-1264 (Mn 8000-12000, OH value 80 mg KOH/1 g resin; from Zeneca).

Isocyanate crosslinker can be di-, tri-, or poly- aliphatic or aromaticisocyanates and adducts between such isocyanates and low molecularweight polyols.

Examples are hexamethylene diisocyanate, isophorone diisocyanate,toluene diisocyanate and 4,4-diphenylmethane diisocyanate. Theisocyanate group in the diisocyanates can be protected or blocked forexample by a caprolactam blocking group commercially available under thetrade mark Vestagon from Hüls.

Polyisocyanates include biuret isocyanurate trimers, allophonates anduretdiones of the diisocyanates listed above. Particular examples arehexamethylene diisocyanate biuret, isocyanurate, allophonate anduretdione/isocyanurate; isophorone diisocyanate isocyanurate andallophonate.

Examples of the low molecular weight polyols that can be used to makeisocyanate/polyol adducts are trimethylol propane, glycerol andpentaerythrithol. The isocyanate crosslinkers described above are knownand can be made by known methods. Many are commercially available underthe trade mark Desmodur.

In an hydroxyalkylamide/carboxy system the carboxy resin can be acarboxy functional polyester or carboxy functional acrylic resin.

Carboxy functional polyester polymers have units derived from one ormore polybasic acids and units derived from one or more polyhydroxycompounds. Resins of this type have been described above with referenceto the isocyanate/hydroxy system.

The carboxy functional polyester polymer are commercially available (egUralac P2127 from DSM Resins BV) and can be produced by known methods.

Carboxy functional acrylic addition polymers are derived from acrylicand methacrylic acids themselves and structural units.

Structural units are derived from monomers which do not have carboxylgroups and will not react with the crosslinker. Examples as describedabove with reference to the isocyanate/hydroxy system. An example of acarboxy functional acrylic resin is Elvacite 2776 available from ICI andhas a Mn of 8000 and acid value of 85 mg KOH/1 g of resin.

An example of an hydroxyalkylamide is Primid XL552 available from Rohmand Haas.

In an acid/epoxy system the epoxy resin can be an aromatic or aliphaticepoxy resin, in particular it can be a glycidyl functional acrylicresin.

Glycidyl functional acrylic addition co-polymers are derived frompolymerisable ethylenically unsaturated monomers such as vinyl oracrylic monomers and comprise functional units and structural units.

Functional units are derived from glydicyl acrylate and methacrylate.

Structural units are as discussed above with reference to theisocyanate/hydroxy system. An example of a glycidyl functional acrylicresin is Almatrex available from Zeneca.

Acid crosslinkers can be C₆-C₁₈ alkanedioic acids. Examples includedodecane-1,12-dioic acid and sebacic acid.

Suitable amino resins include melamine formaldehydes, ureaformaldehydes, benzoguanamines phenol formaldehydes and glycolurils.

Preferably, the resin and crosslinker are selected from epoxy,polyphenol, polyester, blocked isocyanate, hydroxyalkylamidecrosslinker, amino resin, hydroxyl functional polymer and carboxyfunctional polymer or crosslinker.

More preferably, the resin and crosslinker are selected from epoxy,polyphenol, polyester and amino resin.

The liquid carrier for the dispersions of this invention is chosenhaving regard to the temperature at which the process is carried out asdescribed below and such that for practical purposes it is un-reactiveand imiscible with the particles produced. That is to say it does notreact to any major extent with the particles either before thedispersion is applied as a coating composition or before the particlesare separated from the liquid medium. Hence the choice of the liquidmedium will depend on the composition of the particles and the use towhich they will be put.

Where the particles comprise a polybisphenol-A/polyepoxide system, theliquid medium can be water or a liquid hydrocarbon, for example a liquidalkane, in particular, hexane, heptane or octane, but more particularlya high boiling alkane, for example nonane, decane, dodecane andisohexadecane.

Where the particle comprises an isocyanate and a polyol, the liquidmedium can be water in which case the isocyanate should ideally beblocked.

The mixing step is carried out for a time and at a temperature such thatsubstantial crosslinking potential is retained between the resin and thecrosslinker. This is to be understood in a broad practical sense to meanthat after mixing under shear and dispersing in the liquid medium, eachparticle still contains sufficient functional groups in the resin andthe crosslinker to form crosslinked bonds with resin and crosslinkerfrom other particles so as to form a crosslinked network.

Whether or not any particular dispersion meets this criterion can bedetermined by trial and error.

Usually, the particles will contain at least 20 mole % of un-reactedfunctional groups compared to the original level.

For example the particles will contain more than 30 or 40 mole % orpreferably greater than 50 mole % of un-reacted functional groupscompared to the original level.

The temperature at which the mixing is carried out depends upon themelting points or softening points of the resin and crosslinker, theirmutual reactivity, the viscosity of the melt and the speed at whichmixing and dispersion can take place.

The higher the temperature, the more rapidly will any reaction betweenthe resin and the crosslinker take place, so the more rapidly must anymixing and cooling take place to ensure that the particle retainssubstantial crosslinking potential.

Accordingly, where for example a particle is to be made from a resinhaving a relatively high melting point, a slower reacting crosslinkershould be selected; for example a crosslinker where the functionalgroups are protected by blocking groups or are sterically hindered.Alternatively the process can be carried out such that the dispersion israpidly cooled so that the particles solidify and any crosslinkingreaction is quenched.

The liquid carrier is chosen such that its boiling point is above thetemperature at which the process is carried out, except where theprocess is carried out under pressure for example in an extruder.

Examples of typical lower operating temperatures are 50, 60 and 70° C.Examples of typical upper operating temperatures are 200, 250 and 300°C. More usually the temperature will be below 200° C. for example 100 to150° C. However, a preferred temperature range is 70-150° C.

Examples of typical minimum mixing (i.e. processing) times are 15 and 20seconds. Examples of maximum mixing times are 420, 480 and 600 seconds.Usually the mixing time is no less than 20 seconds and no more than 300seconds. For example it is from 30 to 180 seconds. Preferably theprocessing time is in the region 20-240 seconds.

The step of dispersing the melt in the liquid medium is preferablycarried out with the aid of a dispersant. Preferably, the steps ofmelting and mixing the resin and the crosslinker, and step of dispersingare carried out sequentially in a continuous process.

The dispersant helps to disperse the polymer melt in essence mainly bylowering the interfacial tension between the melt and the diluent andsubsequently stabilising the dispersed particles or droplets in theliquid medium. The dispersant can be a block or graft copolymercomprising a stabilising component which is a soluble in the liquidmedium and an anchor component which associates with or reacts with oneor more components in the melt.

The nature of the stabilising component depends on the liquid medium.When the liquid medium is an aliphatic hydrocarbon, the stabilisingcomponent can be an hydrocarbon chain such as a polybutadiene chain.When the liquid medium is water the stabilising component can be apolyethylene glycol or polyvinyl alcohol.

The anchor component can be one which associates with the melt forexample by physical absorbtion or reacts with one or more components init.

Examples of anchor components are polar acrylate and methacrylatepolymers, and moieties. Such dispersants are well known and can be madeby known methods.

The process is controlled such that the size of the droplets of the meltand hence the particles are in the range from 0.1 to 80 microns, forexample 0.1 to 20 microns or 2-50 microns. A preferred range is 1-30microns.

The dispersions generally preferably comprise at least 15% by weight ofparticulate material, for example from 20 to 80% by weight andparticularly from 20 to 60% by weight.

The process is carried out by melting the solid component (usually theresin) or components and mixing them together as a melt with the wholeprocess being carried out in a one stage (i.e. single pass) process, forexample in a single pass through an extruder. The dispersion part of theprocess can be carried out by either direct or inverse emulsification.In the inverse emulsification process, a small amount of carrier liquidmedium and optional dispersant may be introduced and mixed in with themelt forming a dispersion of carrier liquid in the melt. This dispersioncan then be inverted by adding more liquid through a further port,optionally with more dispersant, to form a dispersion of the melt in theliquid. Alternatively the dispersion can be prepared in a single step bydirectly adding a sufficient quantity of carrier liquid and dispersant,followed by mixing.

Preferably the process is carried out in an extruder, and in particulara twin-screw extruder. An example of such an extruder is the Leistritzco-rotating micro-18 GL 40D.

The dispersion so obtained can be used directly as a coating compositionor the particulate material can be separated and used as a powdercoating. Powder particles for powder coating are conventionally solidsat room temperature. Hence the resin and crosslinker should be chosen soas to form a solid once they have been cooled following melt mixing. Theparticles can be separated by centrifugation, sedimentation, filtrationor spray drying. Additionally, the isolated particles may bere-dispersed in another liquid medium and used as a liquid coatingcomposition.

Since the particles have retained crosslinking potential, they can beused to form coatings by applying a mass to a surface for example byspraying or conventional powder application, allowing any liquid carrierto evaporate and stoving the mass to cause the particles to coalesce andto crosslink.

In a coating composition the dispersion or the powder can contain otheradditives standard for coating compositions for example pigments, flowaids and uv-stabilisers.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be further described with reference to thefollowing examples and FIG. 1, which shows an example of a DifferentialScanning Calorimeter trace for a polymer produced according to theinvention.

EXAMPLES Example 1

A solid blend of two different polybisphenol A resins [Araldite GT-7072(49 parts) and Araldite GT-7220 (24 parts)] was prepared with a catalystcontaining epoxy novolac resin [DEH82 epoxy novolac with 3.5% 2-methylimidazole as catalyst (27 parts)], fed at 2 kg hr⁻¹ into the intake feedzone of a Leistritz twin-screw micro 18-GL 40D extruder and melt blendedat 110° C. Downstream of the melt blend step, polyvinyl alcohol (KL-05from Nippon Gohsei) and water (10% non.vol.) was injected at 0.5 kg hr⁻¹into the melt via a liquid injection port whilst maintaining thetemperature of the melt at 110° C., to form an inverse dispersion wherewater is the disperse phase. The inverse dispersion was cooled to 90° C.and then mixed with more water pumped into the extruder at 2 kg hr⁻¹ andextruded through the extruder die to produce an aqueous particulatedispersion. The volume average particle size of the dispersion was 7.7micrometers, as measured by Malvern Mastersizer™.

Example 2

A solid particulate blend of hydroxy acrylic resin powder [Elvacite 2810(70 parts)] and caprolactam-blocked isocyanate crosslinker [VestagonB1530 (30 parts)] was fed at 1 kg hr³¹ ¹ into the intake feed zone of aLeistritz twin-screw micro 18-GL 40D extruder and melt blended at 150°C. Downstream of the melt blend step, styrene/ethyl acrylate/methacrylicacid 40:30:30 latex co-polymer (see Example 4 for preparation method) inwater (30% non.vol.) was injected at 0.6 kg hr⁻¹ followed by ammoniumhydroxide solution (1.4M) at 0.7 kg hr⁻¹ into the melt via sequentialliquid injection port whilst maintaining the temperature of the melt at150° C., to form a dispersion which is thereafter cooled to 90° C. anddiluted with more water pumped into the extruder at 1.3 kg hr⁻¹ andextruded through the extruder die to produce an aqueous particulatedispersion.

The particulate dispersion so obtained was spray dried using a BüchiSpray Dryer to yield a free flowing powder (with a volume averageparticle size of 20.2 microns).

Example 3

The following aqueous dispersions were made according to the protocoldescribed in Example 1 above, but using the following processingconditions and polymer compositions.

Extruder screw Temp. Comp. Polymer composition speed (rpm) (° C.) 1Uralac P2127 (50 parts)/ 250 110 Araldite G7004 (50 parts) 2 DER 6424(70 parts)/DEH 80 250 110 (30 parts) 3 Uralac 2400 (55.7 parts)/ 500 110Primid XL 552 (3.1 parts) 4 Uralac 2400 (80 parts)/ 350 130 AralditePT810 (20 parts) 5 Uralac 2504 (80 parts)/ 350 145 Vestagon B1530 (20parts) 6 Uralac 2127 (52.6 parts)/ 300 145 Primid XL5 (6.2 parts) 7Almatex PD6300 (53 parts)/ 350 145 Sebacic acid (10.1 parts)

Dispersant injection Water injection. Particle Composition Temp.(° C.)Temp.(° C.) Size(*) 1 120 85 1.3 2 120 85 7.1 3 120 85 4.1 4 140 85 1.25 145 85 1.1 6 145 85 2.6 7 145 85 6.7 *-Volume average particle size(measured by Malvern Mastersizer ™), in microns

Solids(% non- Film MEK double Comp. volatiles) Appearance rubs 1 26.8Coalesced 100+ particles 2 25.6 Coalesced 100+ particles 3 22.0Coalesced 100+ particles 4 29.4 Coalesced 100+ particles 5 22.1Coalesced 100+ particles 6 27.3 Coalesced 100+ particles 7 27.9Coalesced 100+ particles

For these compositions, the polymer feed rate was 1-1.2 kg/hour, thedispersant feed rate (20% polyvinyl alcohol solution) was 0.7-1.0kg/hour, and the water feed rate was 2-2.5 kg/hour. The dispersant andwater diluent were added through different injection ports.

The compositions were applied to aluminium panels and stoved at 200° C.for 5 minutes and the solvent resistance of the coating was determinedby the MEK rub test.

The MEK rub test referred to is a standard solvent resistance test whichinvolves rubbing a surface coated with a 20 micrometer thick film of thecompositions with a cloth soaked in methyl ethyl ketone, and measuringthe the number of double finger rubs (a double rub is one forward andone reverse rub) to rub through the film.

Example 4

The following aqueous dispersions were made using a carboxyl functionallatex as the dispersant, according to the protocol described inconjunction with Example 2 above. In this example, the polymer feed ratewas 1.0 kg/hour, the dispersant feed rate (30% latex emulsion) was 0.8kg/hour, the ammonium solution (1.4M) feed rate was 0.6 kg/hour, and thewater feed rate was 1.8 kg/hour. Cymel 1123 was obtained from DynoCyanamid, and Nacure 1419 was obtained from King Industries.

Extruder Screw Melt Comp. Polymer Composition Speed (rpm) Temp(° C.)  8Epikote 1007 (80 parts)/ 350 110 Cymel 1123 (20 parts)  9 Epikote 1004(80 parts)/ 200 120 Cymel 1123 (20 parts)/ Nacure 1419 (0.2 parts) 10Araldite GT 7072 (2 parts)/ 200 120 Araldite GT 7220 (1 part)/ Dow DEH82 (1 part) 11 Araldite GT 7072 (2 parts)/ 300 110 Araldite GT 7220 (1part)/ Dow DEH 82 (1 part)

Dispersant- Injection Water injection. Particle Comp. Temp. (° C.)Temp.(° C.) Size(*)  8 120 85 17  9 120 85 22 10 130 80 2.3 11  95 802.0 *Volume average particle size (microns, as measured by MalvernMastersizer ™)

Solids (% non- Film MEK double Comp. volatiles) Appearance rubs  8 24Coalesced 65 particles  9 25 Coalesced 70 particles 10 33 Partially 65coalesced particles 11 30 Coalesced 100+ particles

The latex used was a styrene/ethylacrylate/methacrylic acid (40/30/30)latex, with a calculated T_(g) of 73° C. and an acid value of 194 mgKOH/gram of polymer. This was prepared as follows.

Material Weight (g) Aqueous charge Deionised water 4015.2 Aerosol OT7511.2 Monomer feed Styrene 1382.7 Ethyl acrylate 1037.0 Methacrylic acid1037.0 Aerosol OT75 22.4 Primary octyl mercaptan 68.8 Seed InitiatorDeionised water 124.8 Ammonium persulphate 17.6 Feed Initiator Deionisedwater 248.8 Ammonium persulphate 34.4

In the preparation method, the aqueous charge is heated to 80° C. undera nitrogen blanket, and 10% of the monomers are added. This is then heldfor 10 minutes. A shot of the seed initiator is then added, and thetemperature raised to 85° over a period of 30 minutes. Then the monomerand initiator are fed in linearly over 3 hours, and the mixture is thenheld at 85° C. for 1 hour. The composition is then cooled to roomtemperature and filtered. The result is a fluid milky latex, which wasused as a dispersant after being neutralised in the dispersion process.

Example 5

The following pigmented aqueous dispersions were made according to theprotocol generally described above in conjunction with example 2. Inthis example, the polymer feed (Elvacite 2810 (40 parts)/Vestagon B1350(17 parts)/TiO₂ TR-60 (39 parts)) rate was 1 kg/hour, the dispersant(30% latex emulsion) feed rate was 0.5 kg/hour, the ammonium solution(1.4M) feed rate was 0.6 kg/hour, and the water feed rate was 2.0kg/hour.

The extruder screw speed was 250 rpm, the melt temperature was 150° C.,and the dispersant injection temperature was 150° C. The water injectiontemperature was 90° C. The average volume particle size was 0.4 microns,which was thought in part to be due to the presence of the titaniumdioxide, and was actually 2 microns when measured by optical microscopy.The solids content (% non-volatiles) was 26.2.

The particulate dispersion so obtained was spray dried using a BuchiSpray Dryer to yield a free flowing powder with a volume averageparticle size of 14 microns, with this in part being due to particleagglomeration which has occurred during the spray drying process.

Example 6

The following non-aqueous dispersion was made according to the protocoldescribed in example 1. The following process conditions were used. Thepolymer feed rate was 2.0 kg/hour, and the dispersant (10%polybutadiene-graft-acrylic copolymer in isohexadecane) feed rate was1.0 kg/hour, and the isohexadecane (ex. Bayer) feed rate was 2 kg/hour.

The polymer composition was Araldite GT 7072 (2 parts), Araldite GT 7220(1 part) and Dow DEH 82 (1 part). The extruder screw speed was 250 rpm,the melt temperature was 120° C., and the dispersant injectiontemperature was 130° C. The isohexadecane injection temperature was 120°C., the average volume particle size was 15.1 microns, and the % nonvolatile solids was 44.

The dispersant in this example was made as follows.

Parts Charge 1 Toluene 396.14 Polybutadiene (Lithene N4 5000, ex.Revertex) 199.74 White Spirit 197.74 Charge 2 Methyl methacrylate 187.74Methacrylic acid  11.98 Lucidol P25 (25% aq benzoyl peroxide)  5.33

Charge 2 was added to charge 1 over a period of 1.5 hours at a refluxtemperature of 120-125° C. After a further 30 minutes, Trigonox 21B 70(tertiary butyl per-2-ethylhexanoate, ex. Ciba Chemicals, 1.33 parts)was added, and heating was continued for a further hour. Solvent (99.1parts) was removed by distillation, and replaced by an equal volume ofwhite spirit.

The product was opalescent, with a viscosity of 0.5-1.0 Pas and ameasured solid content of 37%

Example 7

Compositions 1-11 above were further examined using DifferentialScanning Calorimetry (DSC) and data is given in table below. Accordingto the test method, the sample polymer is accurately weighed into analuminium DSC pan. The lid of this is pierced four times, and the lidand pan assembly are crimped together. The crimped pan is placed intothe cell of the DSC (TA Instrument 2000), and scanned from −50° C. to300° C. at a temperature change rate of 10° C. per minute. Thethermogram for the Tg step transition is analysed, and the area of thereaction peak integrated. The DSC trace for Composition 11 is shown asFIG. 1. FIG. 1 indicates that the polymer of Composition 11 has a Tg of55.3° C. and a heat of reaction of 87.2 J/g attributable to residualreactivity.

Heat of reaction Composition of extrudate (J/g) 1 34.8 2 69.8 3 (Notdetected) 4 10.5 5 14.3 6 36.6 7 42.8 8  6.4 9 14.4 10  45.2 11  87.2

Composition 3 does not appear to lend itself to this method of analysis.Compositions 10 (processed at relatively high temperature) and 11(processed at lower temperature) show the influence of processtemperature upon the measured heat of reaction, and this is alsoreflected in the quality of the films, with composition 10 yielding afilm of partially coalesced particles.

Example 8

It was attempted to produce the following comparative composition.

A solid blend (80 parts) of two resins and a cross linker (Araldite GT7072, 49 parts, Araldite GT 7220, 24 parts, and Dow DEH 82, 27 parts)together with butyl Cellosolve (50 parts) were heated in a round bottomflask fitted with a mechanical stirrer. The temperature was raised to70° C.

To this mixture a solution of acid functional acrylic polymer in butylcellosolve (at 60% nv) was added (30 parts), with vigorous stirring. Theacrylic solution polymer was prepared in a manner similar to thatdescribed in U.S. Pat. No. 5,087,645 referred to above,and had acomposition of methylmethacrylate/butyl acrylate/acrylic acid with aratio 46.2:44.4:9.3 respectively.

To the above mixture dimethylaminoethanol (2 parts) was added, and themixture was continuously agitated vigorously. Afterwards water (37parts) was added to yield a milky dispersion, and it was then cooled byplacing an ice bath underneath the flask.

The dispersion was left overnight to allow most of the particles tosediment, forming a layer at the bottom. This bottom layer of productwas isolated, and left to dry under ambient conditions. It was foundthat the resulting product was a continuous resinous mass, which couldnot be transformed into a free flowing powder state, due to the presenceof solvents used in the processing steps.

In a further comparative example, a solvent free version of the abovecomparative example was attempted. It was found that upon heating thesolid resins/crosslinker blend (Araldite GT 7072, 49 parts, Araldite GT7220, 24 parts, Dow DEH 82, 27 parts) the system crosslinked and becameimpossible to process.

What is claimed is:
 1. A continuous process for preparing a reactiveparticulate dispersion in a liquid carrier which comprises mixingtogether under extrusion conditions to form a molten mixture a resin anda crosslinker under shear at a temperature and for a time such thatsubstantial crosslinking potential is retained between the resin and thecrosslinker and thereafter dispersing the mixture whilst still molteninto the liquid carrier and allowing the molten dispersed mixture tosolidify to form particles.
 2. A process according to claim 1, whereinthe process is carried out in the absence of a volatile organic solvent.3. A process according to claim 1 or claim 2, wherein a liquid carrieris used which is immiscible with the particles.
 4. A process accordingto any one of the preceding claims wherein the resin and crosslinker areselected from epoxy, polyphenol, polyester, blocked isocyanate,hydroxyalkylamide crosslinker, amino resin, hydroxyl functional polymerand carboxy functional polymer or crosslinker.
 5. A process according toclaim 4 in which the resin and crosslinker are selected from epoxy,polyphenol, polyester and amino resin.
 6. A process according to any oneof the preceding claims, wherein the liquid carrier is water or ahydrocarbon.
 7. A process according to any one of the preceding claims,wherein the step of melting and mixing the resin and the crosslinker andthe step of dispersing are carried out sequentially in a continuousprocess.
 8. A process according to any one of the preceding claims,wherein the melting, mixing and dispersing steps are carried out in anextruder.
 9. A process according to claim 7 wherein the extruder is atwin screw extruder.
 10. A process according to any one of the precedingclaims, wherein the particles are subsequently separated from the liquidcarrier.
 11. A dispersion of reactive particles in a liquid carrier,wherein the dispersed particles comprise reactive particles of a resinand a crosslinker, wherein the particles have a blocking temperature ofat least 40° C., are free of a solvent in which the resin is miscible,and are substantially spherical, wherein the resin/crosslinker system isan acid/epoxy, epoxy/amino resin, polyphenol/epoxy, phenolformaldehyde/epoxy, epoxy/amide, isocyanate/hydroxy,carboxy/hydroxyalkylamide or hydroxyl/epoxy system, said particlescomprising a free-flowing powder upon removal of said carrier.
 12. A dryfree-flowing powder comprising dispersed particles prepared by a processcomprising mixing together under extrusion conditions to form a moltenmixture a resin and a crosslinker under shear at a temperature and for atime such that substantial crosslinking potential is retained betweenthe resin and the crosslinker and thereafter dispersing the mixturewhilst still molten into a liquid carrier and allowing the moltendispersed mixture to solidify to form particles, wherein the particlesare subsequently separated from the liquid carrier.
 13. A continuousprocess for preparing a reactive particulate dispersion in a liquidcarrier in an extruder equipped with a main intake, an exit port, and anintermediate liquid injection port between the main intake and the exitport, and heating means for heating material as it passes through theextruder between the main intake and the liquid injection port, in whicha resin and a cross linker are introduced into the extruder through themain intake, and are heated and mixed together as they pass through theextruder so as to form a molten mixture before they reach the liquidinjection port, and a liquid carrier is introduced into the extruderthrough the injection port, and the molten mixture becomes dispersed inthe liquid medium, the temperature and the throughput of the extruderbeing such that substantial crosslinking potential between the resin andthe crosslinker remains in the final dispersion, and the dispersion isallowed to cool on leaving the extruder such that the molten mixturesolidifies to form a particulate dispersion.
 14. A dispersion accordingto claim 11 wherein each of the reactive particles comprises ahomogeneous mixture of resin and crosslinker throughout the particle.15. A dry free-flowing powder comprising substantially sphericalparticles obtained by removal of the carrier liquid from the dispersionof claim 11.